tree-widget-themes-directory

tree-widget-themes-directory is a variable defined in `tree-widget.el'.
Its value is

"tree-widget"

Documentation:
Name of the directory in which to look for an image theme.
When nil use the directory where the tree-widget library is located.
When it is a relative name, search in all occurrences of that sub
directory in the path specified by `tree-widget-themes-load-path'.
The default is to use the "tree-widget" relative name.

You can customize this variable.